Antenna elements may be individual antennas or patches, for example, which are offset with respect to the optical axis of a common radar lens. The directional characteristic of each antenna element, specifically the direction in the greatest radiation intensity or the greatest sensitivity, is then given by the offset of the respective element with respect to the optical axis. Alternatively, however, the antenna elements may be so-called phased arrays made up of multiple sub-elements, which are supplied with transmission signals having such a phase relationship that the desired directional characteristic results by interference. The same antenna elements or alternatively separate antenna elements may be used for transmitting and for receiving the radar signals.
Such radar sensors are used, for example, in so-called ACC systems (adaptive cruise control) for motor vehicles and serve the purpose of measuring the distances and relative speeds of preceding vehicles so as to allow for an adaptive distance control and speed control. A certain angular resolution capability of the radar sensor makes possible the determination of the azimuth angle of the located objects, so that, for instance, one may distinguish between preceding vehicles in one's own lane and vehicles in side lanes.
As an example of such a radar sensor, European Patent Application No. EP 1 380 854 A2 describes a static FMCW multibeam radar. In this connection, the term “static” signifies that the directions of the radar beams generated by the individual antenna elements are invariable over time so that the entire locating angle range of the angular resolution radar sensor may be monitored simultaneously by parallel evaluation of the signals supplied by the individual antenna elements.
In an FMCW radar (frequency modulated continuous wave), the frequency of the transmission signals supplied to the individual antenna elements is modulated in ramp-shaped fashion. The signal received from each individual antenna element is mixed with the transmission signal that is supplied to this antenna element. In this manner, an intermediate-frequency signal is obtained, the frequency of which indicates the frequency difference between the transmitted signal and the received signal. This frequency difference is a function of the relative speed of the located object on account of the Doppler effect, but is also a function of the signal propagation time, and thus of the distance of the object, due to the modulation of the transmitted signal.
The intermediate frequency signals are digitized and recorded over a time span approximately corresponding to one individual frequency ramp. The signal pattern thus obtained is then split up into its frequency spectrum by fast Fourier transform. In this spectrum, each located object emerges as one individual peak, the frequency position of which is a function of the distance and the relative speed of the respective object. If the transmitted signals are alternately modulated using frequency ramps having different ramp slopes, for example having a rising ramp and a falling ramp, then it is possible definitely to determine, for an individual object, the distance and the relative speed of the object from the position of the peaks in the spectrums obtained for the two ramps. If multiple objects are located simultaneously, then it is necessary, for a definite allocation of the peaks to the respective objects, to modulate the transmitted signals using at least one additional frequency ramp.
For each channel, i.e., for each antenna element, a spectrum is obtained on each frequency ramp, in which the located objects emerge in the form of a peak. For the peaks pertaining to an individual object, the amplitude and the phase of the intermediate frequency signal, for example at the apex of the peak, differ somewhat from channel to channel. The differences in the amplitude and phase, collectively also known as a complex amplitude, result from the different directional characteristics of the antenna elements and are a function of the azimuth angle of the respective object.
For each individual antenna element, the complex amplitude displays a characteristic dependence on the azimuth angle, which may be represented in an antenna diagram. The distance and the relative speed of the object enter into the complex amplitude only in the form of a factor, which is identical for all channels. By comparing the complex amplitudes in the different channels, it is thus possible to determine the azimuth angle of the respective object. Stated in simplified terms, the azimuth angle is sought at which the complex amplitudes measured respectively at the apex of the peak fit best with the associated antenna diagrams. In the radar sensor described in European Patent Application No. EP 1 380 854 A2, in order to improve the angular resolution, the complex amplitude is evaluated not only at the apex of the respective peak, but at multiple frequencies in proximity to this apex.
In the conventional radar sensor, the same frequency-modulated transmission signal is supplied to all antenna elements. As an example, let it be assumed that the same antenna elements are used for transmission and for reception. Each antenna element then receives a radar echo, not only from the signal that it had sent itself, but also from the signals sent by the other antenna elements. Provided they come from the same object, all these signals have the same frequency and are superposed on each other on the receiving antenna element to form a composite signal. Now, if, for example, two objects which differ in their azimuth angle, but have the same distance and the same relative speed, then their signals can no longer be separated in the spectrum so that the radar sensor is unable to resolve the different azimuth angles of the two objects.
Another effect that impairs the angular resolution capacity of the known radar sensor results from the fact that the individual antenna elements do not generate sharply bundled beams, but rather relatively widely fanned radar lobes, due to diffraction and interference effects. Typically, two or more secondary lobes form in addition to a main lobe. The form and intensity of the main lobes and the secondary lobes are influenced by the coupling with the equal-frequency signals transmitted by other antenna elements.
In the case of advanced ACC systems which are also to be used, for example, in urban traffic or in stop and go operation in a traffic jam, the detailed recording of the traffic environment is required in the near field. The FMCW radar sensor described above is generally not sufficient for this purpose, because in the near field the radar lobes are not yet fanned sufficiently widely, so that objects that are offset laterally are not able to be located. Up to now, it has therefore been necessary to provide additional sensoric components for the near-field, to supplement the FMCW radar sensor, or to switch over between strongly and weakly bundling antennas or activation patterns for the phased arrays, which requires a relative complex design.